This invention relates to methods and apparatus for correlating two time series of digital quantities.
The use of swept frequency acoustic signals is well known in the geophysical prospecting art. A basic disclosure of such use is contained in U.S. Pat. No. 2,688,124 entitled "Method of and Apparatus for Determining Traveltime of Signals" issued to Doty et al. on Aug. 31, 1954. As disclosed in this Doty patent, low energy level seismic waves may be used effectively for geophysical prospecting if the frequency of the seismic wave is swept according to a known pattern and the reflection signals detected by geophones at the surface are crosscorrelated with this known pattern or pilot signal. Standard practice in geophysical prospecting is to record the outputs of geophones at the earth's surface and to perform the crosscorrelation sometime later, and typically in a large general purpose computer.
The output of a crosscorrelator in a swept frequency geophysical prospecting system is an earth spikes trace similar to the outputs of impulse type geophysical prospecting systems. One trained in the geophysical prospecting art can derive considerable information by visual examination of an earth spikes trace, but has great difficulty in learning anything from a visual examination of a direct output of a geophone in a swept frequency system. A visual examination of an earth spikes trace can be quite useful in the field to allow a technician to know if the equipment is operating properly and for a geophysicist to identify areas of interest which might warrant further exploration before leaving the field location. Thus it is seen that it is desirable to include a correlator in the field portion of a swept frequency seismic system.
Since standard correlation requires the use of large general purpose computers, or expensive special purpose computers, they generally have not been included in field portions of swept frequency geophysical systems. Disclosures of correlation apparatus suitable for field use may be found in U.S. Pat. No. 3,863,057 entitled "Apparatus for Serially-Correlating Time Series" issued to Siems on Jan. 28, 1975, and U.S. Pat. No. 3,863,058 entitled "Apparatus for Digital Correlation" issued to Savit on Jan. 28, 1975. The correlator disclosed by Siems is smaller than a standard correlator because it ignores the mantissa of pilot and data samples and multiplies only the signed exponent of each pair of samples to generate a correlation between pilot and data. In the Savit correlator, the multiplication step of the standard correlator is eliminated by again ignoring the mantissas of the samples and adding the exponents of pairs of pilot and data samples. Each sum is exponentiated to obtain the equivalent product which is then summed with other products to generate the final correlation. Thus, it is seen that both Siems and Savit reduce correlator complexity by reducing the number of digital bits which represent each sample and Savit additionally reduces complexity by avoiding the multiplication step but does so at the cost of adding digital exponentiators. But both of these correlators still require that mathematical operations be performed upon every sample contained in the shorter of the two time series of samples for each correlation.